1. Field of the Invention
The present invention relates to an antiparallel spin valve read head with improved magnetoresistance and biasing and, more particularly, to an antiparallel pinned spin valve read sensor which has a free layer structure that includes a cobalt or cobalt based layer that interfaces a spacer layer for improved magnetoresistance and a sizing and positioning of layers of the spin valve sensor for minimizing readback asymmetry.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk.
The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. In order to improve the sensitivity of the spin valve sensor a soft magnetic material, such as nickel iron (NiFe), is employed as the free layer. It has been found, however, that when a free layer structure employs a cobalt based layer in addition to the nickel iron (NiFe) free layer that the magnetoresistive coefficient dr/R increases when the cobalt based layer is located between and interfaces the nickel iron (NiFe) free layer and a copper (Cu) spacer layer. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
An improved spin valve, which is referred to hereinafter as antiparallel pinned (AP) spin valve, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. The AP spin valve differs from the spin valve described above in that the pinned layer comprises multiple thin films, hereinafter referred to as AP pinned layer. The AP pinned layer has a nonmagnetic spacer film which is sandwiched between first and second ferromagnetic thin films. The first thin film, which may comprise several thin films, is immediately adjacent to the antiferromagnetic layer and is exchange-coupled thereto, with its magnetic moment directed in a first direction. The second thin film is immediately adjacent to the free layer and is exchange-coupled to the first thin film by the minimal thickness (in the order of 6 xc3x85) of the spacer film between the first and second thin films. The magnetic moment of the second thin film is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first film. The magnetic moments of the first and second films subtractively combine to provide a net moment of the AP pinned layer. The direction of the net moment is determined by the thicker of the first and second thin films. The thicknesses of the first and second thin films are chosen so that the net moment is small. A small net moment equates to a small demagnetization (demag) field from the AP pinned layer. Since the antiferromagnetic exchange coupling is inversely proportional to the net moment, this results in a large exchange coupling.
A large exchange coupling promotes higher thermal stability of the head. When the head encounters high heat conditions due to electrostatic discharge from an object, or due to contacting an asperity on the magnetic disk, a critical high temperature of the antiferromagnetic layer, hereinafter referred to as blocking temperature, can be exceeded, causing the magnetic spins of the pinning layer to be free to rotate in response to a magnetic field. The magnetic moment of the AP pinned layer is then no longer pinned in the desired direction. In this regard, significant advantages of the AP pinned spin valve over the typical single film pinned layer are a greater exchange coupling field and a lower demag field, which enhance thermal stability of the spin valve sensor.
As stated hereinabove, the AP pinned layer structure of the spin valve sensor imposes less demagnetization field HD on the free layer structure. This is important because a demagnetization field from a pinned layer structure, whether it be a simple single pinned layer or an AP pinned layer structure, is not uniform between the ends of the pinned layer structure that are perpendicular to the ABS. The demagnetization field is strongest at the ends and decays toward the middle of the sensor due to the first and second shield layers. This causes a nonuniform biasing of the free layer structure that impacts the sensitivity of the read head. Further, the demagnetization field HD is a function of the stripe height of the sensor wherein the stripe height is the distance between the ABS and an opposite recessed end of the sensor in the read head. The reason for this variation is because of the difficulty in controlling the lapping of various rows of magnetic head assemblies to establish their stripe heights. Unfortunately there is a sigma (distribution of stripe heights) from row to row and between the magnetic head assemblies from row to row. Accordingly, magnetic heads from one row of magnetic heads may have positive readback asymmetry while magnetic heads from another row of magnetic heads may have a negative readback asymmetry. Since the demagnetization field from the AP pinned layer structure is significantly less than that from a simple pinned layer the aforementioned sigma and degree of nonuniform demagnetization field HD acting on the free layer structure is minimized.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced.
Readback asymmetry is defined as                     V        1            -              V        2                    max      ⁡              (                              V            1                    ⁢                      xe2x80x83                    ⁢          or          ⁢                      xe2x80x83                    ⁢                      V            2                          )              .
For example, +10% readback asymmetry means that the positive readback signal V1, is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in many applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. In these applications +10% readback asymmetry can saturate the free layer in the positive direction and will, of course, reduce the negative readback signal by 10%. An even more subtle problem is that readback asymmetry impacts the magnetic stability of the free layer. Magnetic instability of the free layer means that the applied signal has disturbed the arrangement or multiplied one or more magnetic domains of the free layer. This instability changes the magnetic properties of the free layer which, in turn, changes the readback signal. The magnetic instability of the free layer can be expressed as a percentage increase or decrease in instability of the free layer depending upon the percentage of the increase or decrease of the asymmetry of the readback signal. Standard deviation of the magnetic instability can be calculated from magnetic instability variations corresponding to multiple tests of the free layer at a given readback asymmetry.
There is approximately a 0.2% decrease in standard deviation of the magnetic instability of the free layer for a 1% decrease in readback asymmetry. This relationship is substantially linear which will result in a 2.0% reduction in the standard deviation when the readback asymmetry is reduced from +10% to zero. Magnetic instability of the free layer is greater when the readback asymmetry is positive. Accordingly, the magnetic instability of the free layer is greater when the readback asymmetry is positive than when the readback asymmetry is negative. A positive readback asymmetry can be improved by changing thickness of the magnetic layers and/or changing the sense current, however, a change of one of these parameters can change other parameters making it a trial and error process to reduce the asymmetry. More importantly, however, is that the channel electronics of the disk drive as well as specific thicknesses of the magnetic layers are designed to satisfy other magnetic considerations. Since these values are set it is manifest that there is a need to deal with the many magnetic influences on the free layer of the AP pinned spin valve so that the net value of these influences on the free layer can be reduced to virtually zero thereby reducing the asymmetry to virtually zero.
A spin valve sensor has a magnetoresistive (MR) coefficient which is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. In the AMR sensor changes in the scattering of the conduction electrons changes the resistance of the AMR sensor proportional to cos2 xcex8 where xcex8 is the angle between the directions of a sense current conducted through the AMR sensor and the moment of a ferromagnetic layer. The resistance of the AMR sensor is maximum when the directions of the sense current and the moment of the ferromagnetic layer are parallel or antiparallel and the resistance of the AMR sensor is minimum when the these directions are perpendicular. The AMR effect in the AMR sensor is also present in the spin valve sensor and its affect on the bias point can be defined in terms of magnitude and direction. The AMR effect on a spin valve sensor always causes negative asymmetry because the resistance change of the spin valve sensor in response to the positive and negative signal fields is always positive which causes a positive bias on the magnetic moment of the free layer.
When the sense current Is is applied to the spin valve sensor there is an image sense current in each of the first and second shield layers. The image sense current in each shield layer causes each shield layer to produce an image sense current field HIM which traverses the free layer in a direction that is substantially perpendicular to the ABS. When the free layer of the AP pinned spin valve is symmetrically located midway between the first and second shield layers the image sense current fields counterbalance each other so that the net image sense current field on the free layer is zero. By asymmetrically locating the free layer between the first and second shield layers a net image sense current field can be employed for counterbalancing the net magnetic and AMR effect influences on the free layer. This is accomplished by sizing the first and second gap layers that separate the free layer from the first and second shield layers respectively so that the free layer is closer to a selected one of the shield layers. It is preferred that the second gap be thinner than the first gap so that the free layer is closer to the second shield layer. When these thicknesses are carefully selected readback asymmetry can be improved so that magnetic stability of the free layer is optimized.
The location of the transfer curve relative to the bias point is influenced by five major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demag field HD from the pinned layer, a sense current field HI from all conductive layers of the spin valve except the free layer, a net image current field HIM from the first and second shield layers and the influence of the AMR effect. There is a need to deal with these forces on the free layer so as to improve asymmetry of the readback signals.
The bias point of an AP pinned layer structure, as described hereinabove, without the cobalt or cobalt based layer in the free layer structure can be balanced with the AMR effect HAMR and the image current field HIM opposing the ferromagnetic coupling field HFC, the sense current field HI and the demagnetization field HD. However, when the cobalt or cobalt based layer is included in the free layer structure the AMR effect HAMR becomes negligible and the image current field HIM is not sufficient to counterbalance the ferromagnetic coupling field HFC, the sense current field Hand the demagnetization field HD. I have overcome the aforementioned problem by making the second AP pinned layer of the AP pinned layer structure thicker than the first AP pinned layer wherein the second AP pinned layer interfaces the spacer layer. In a bottom type spin valve sensor the first read gap is greater than the second read gap so that an imaging sense current HIM from the second shield layer is controlling and is in the same direction as the ferromagnetic coupling field HFC. With the sense current being conducted in either direction through the sensor the net demagnetization field HD from the AP pinned layer structure and the sense current field HI will counterbalance the ferromagnetic coupling field HFC and the image current field HIM. With this arrangement the bias point can be located midway on the transfer curve of the sensor for establishing readback symmetry.